Various types of compound semiconductor devices are known. Among them, a high electron mobility transistor (HEMT) produces less noise and can operate at high speed, and is used in base stations for cellphones because of these characteristics.
In a HEMT, two dimensional electron gas is generated in a channel layer due to a difference in spontaneous polarization between the channel layer and an electron supply layer disposed over the channel layer. It is supposed that the power of a HEMT can be increased by increasing the density of the two dimensional electron gas.
However, it has been reported that the electron mobility decreases when the density of two dimensional electron gas is increased. Accordingly, it is difficult to increase the power of a HEMT by increasing the density of two dimensional electron gas (see, for example, M. N. Gurusinghe et al., “Two-dimensional electron mobility limitation mechanisms in AlxGa1-xN/GaN heterostructure”, Physical Review B 72, 045316 (2005)).